1. Field of the Invention
This invention relates to a novel process and reactor for catalytically dewaxing a waxy liquid petroleum feedstock utilizing a shape-selective crystalline zeolite catalyst. More particularly, this invention relates to improvements in the shape-selective dewaxing of liquid petroleum feedstocks to obtain improved product quality, operating efficiency, as well as improvements in the aging characteristics of the shape-selective dewaxing catalyst.
2. Discussion of the Prior Art
The cracking and/or hydrocracking of petroleum stocks is in general well known and widely practiced. It is known to use various zeolites to catalyze cracking and/or hydrocracking processes. The cracking may have the intent to convert a certain class of compounds in order to modify a characteristic of the whole petroleum stock. Exemplary of this type of conversion is shape-selective conversion of straight and slightly branched aliphatic compounds of 12 or more carbon atoms to reduce pour point, pumpability, and/or viscosity of heavy fractions which contain these waxy constituents. The long carbon chain compounds tend to crystallize on cooling of the oil to an extent such that the oil will not flow, hence may not be able to be pumped or transported by pipelines. The temperature at which such mixture will not flow is designated the "pour point", as determined by standarized test procedures.
The pour point problem can be overcome by techniques known in the art for removal of waxes or conversion of those compounds to other hydrocarbons which do not crystallize at ambient temperatures. An important method for so converting waxy hydrocarbons is shape-selective cracking or hydrocracking utilizing principals described in U.S. Pat. No. 3,140,322 dated July 7, 1964. Zeolitic catalysts for selective conversions of wax described in the literature include such species as mordenite, with or without added metal, to function as a hydrogenation catalyst.
Particularly effective catalysts for catalytic dewaxing include zeolite ZSM-5 and related porous crystalline aluminosilicates, as described in U.S. Re. Pat. No. 28,398 (Chen et al) dated Apr. 22, 1975. As described in that patent, drastic reductions in pour point are achieved by catalytic shape-selective conversion of the wax content of heavy stocks with hydrogen in the presence of a dual-functional catalyst of a metal plus the hydrogen form of ZSM-5. The conversion of waxes is by scission of carbon-to-carbon bonds (cracking) and production of products of lower boiling point than the wax. However, only minor conversion occurs in dewaxing. For example, Chen et al describe hydrodewaxing of a full range shale oil having a pour point of +80.degree. F. (+27.degree. C.) to yield a pumpable product of pour point at -15.degree. F. (-26.degree. C.). The shift of materials from the fraction heavier than light fuel oil to lighter component was in the neighborhood of 9% conversion.
Current technology for dewaxing petroleum stocks having elevated pour points involves the use of trickle beds, whereby gas (primarily hydrogen) and the petroleum stock concurrently flow downward over a bed of solid catalyst. This three-phase trickle bed concept makes use of an intimate mixing between gas and liquid phases while in contact with the catalyst in order to facilitate dewaxing. Performance level of the process is gauged by the length of time during which the process is producing products which meet specifications, as well as the minimum temperature required to attain acceptable products. A trickle bed process is disclosed in U.S. Pat. No. 4,332,670 to Antal, which is directed to hydrodewaxing a distillate hydrocarbon compound over a zeolite ZSM-5 containing catalyst. In the trickle bed reactor the gas is the continuous phase, while the liquid petroleum stock trickles down as a discontinuous phase.
In the hydroprocessing of liquid petroleum feedstocks, including heavy feedstocks, e.g., 650.degree. F.+ (343.degree. C.+) lube oils, it is desired to remove lighter conversion products from the liquid petroleum flow while concurrently providing a hydrogenative environment for catalytic conversion, which is particularly utilized in the case of the processing of highly waxy feedstocks. Frequently, lighter products which are obtained from cracking and/or hydrocracking reactions compete with the heavier feed molecules for access to the cracking sites in the zeolites which are employed in the implementation of the catalytic dewaxing process. Inasmuch as these lighter products diffuse more rapidly into the catalyst than the larger feed molecules, they have a tendency to retard the rate of conversion of the heavier molecules. Moreover, the lighter products also tend to be either more difficult to crack, such as low molecular weight paraffins, or easier to polymerize, such as low molecular weight olefins. They also possess a tendency to coke more readily than their heavier counterparts so as to thereby retard the conversion of the heavier molecules to an even greater extent.
This competition between the light and heavy liquid petroleum molecules is rendered particularly critical when there is employed a catalyst which essentially constitutes a shape-selective zeolite, for example, a zeolite exemplified by ZSM-5, for the dewaxing of liquid petroleum or lube stocks. Processes in reactors which utilize aluminosilicate zeolite catalysts, such as ZSM-5 or other zeolites having smaller pore openings, are disclosed in U.S. Pat. No. 4,222,855 to Pelrine et al and in U.S. Re. Pat. No. 28,398 to N. Y. Chen, both of which are incorporated herein by reference. U.S. Pat. No. 4,263,126 to Rollmann discloses dewaxing a hydrocarbon oil by use of a powdered ZSM-5 catalyst dispersed in the oil undergoing conversion.
Other techniques for catalytic hydrodewaxing involve the use of ebullated catalyst beds. U.S. Pat. No. 2,987,465 is directed to a batch process for contacting a liquid and a gas in the presence of an ebullating catalyst bed. The catalysts are maintained in random motion by up-flowing streams of liquid and gas which enter the bottom of the reactor. U.S. Pat. No. 2,706,167 to Harper is directed to the hydrogenation of hydrocarbon oils, in which the feedstock and hydrogen are introduced into the lower part of the reactor with force enough to suspend and ebullate the catalyst during the reaction. U.S. Pat. No. 4,317,746 to Richardson is directed to the use of rotating mixers to maintain catalyst particles in a state of suspension in an ebullating bed.
Although the utilization of different types of hydroprocessing reactors to implement catalytic dewaxing processes is disclosed in the prior art, as exemplified by the above-mentioned U.S. patents, broadly referring to ebullated reactors, stirring tank-type reactors or trickle bed reactors, there is an obvious need in the technology to more precisely define specific hydroprocessing reactor designs. Needed are reactors which, in a highly efficient and novel manner, will facilitate the removal of the lighter products from the liquid petroleum feedstocks through the intermediary of hydrogen stripping, while concurrently preserving an intimate three-phase contacting relationship among hydrogen, liquid petroleum feedstock and catalyst.
Another problem which has not been adequately solved during dewaxing of lubricating oil feedstocks with shape-selective zeolite catalysts is the aging of the dewaxing catalyst. Thus, it has been found that as dewaxing proceeds, a higher reactor temperature is required to compensate for decreased catalytic activity and meet product specifications. However, increasing the reactor temperature incurs additional energy costs, often changes the product distribution which is obtained, and often results in increased coking of the feedstock during dewaxing. Coking further decreases the catalyst life.
Accordingly, another object of the present invention is to reduce the aging rate of the shape-selective dewaxing catalyst during catalytic dewaxing of a lubricating oil or other waxy liquid petroleum feedstock.